The present invention relates to systems for determining the direction of origin of an electromagnetic signal.
It is often desirable to determine from where a radio, data transmission or other signal is originating, especially in military applications. Direction finding (DF) systems receive a signal at a number of antennas connected to one or more receivers or receiver channels (one receiver may have many channels). The antenna signals are compared, usually by comparing the phase, so that a calculation can be done to determine the direction of origin of the signal. Usually more than one receiver is needed to process the signals from the different antennas or elements.
Since very small variations in the signal are being measured, the two or more receivers (or receiver "channels") must be very closely matched so that errors are not introduced by the receiver itself. This requires special IF (intermediate frequency) filters that must match each other. This is expensive even for two channels, and as more channels are added, it becomes even more expensive, especially when a failed unit must be replaced.
Recent advances in semiconductor technology have allowed various algorithms for calculating signal spectra to be used to process signals in the digital domain. One of these algorithms is the Fast Fourier Transform (FFT), and it has been suggested in the literature that an FFT phase interferometer direction-of-arrival system could be used and could correct for phase differences between channels in the digital domain, eliminating the need for matched analog filters. Such a discussion is contained in a treatise entitled "FFT Signal-Processing and System Applications", by E. Oran Brigham, published in 1988 by Prentice-Hall. A document entitled "Dual Channel Space Quadrature Inferometer System" by Peter Floyd and James Taylor, and distributed by Watkins-Johnson Company in 1986, discusses "chirping" a signal to do the calibration. In other words, one frequency after another is sequentially applied to each receiver channel, with the calibration being done for each frequency.